Generally, in continuous molten metal plating and continuous painting, a so-called gas wiping is widely adopted in which an excessive amount of coating material such as a molten metal, paint, etc. is continuously coated on the surface of a strip and at the same time blows of gas are applied to the surface by use of a wiping nozzle to remove excessive coating material.
In the above-described continuous molten metal plating and continuous painting, it is a matter of great importance to accurately adjust the coating weight of a coating material such as the molten metal, paint, etc. on a strip to a target value.
The adjustment of the coating weight of a coating material on a continuous molten metal plating line and a painting line is subdivided into many types from the points of diversified purposes, corrosion resistance, and cost of products. Therefore, not only when the type of product is to be changed but when the target coating weight is to be changed, it is necessary to properly and rapidly change and set the nozzle injection pressure P, nozzle-strip distance D, slit clearance B of a wiping nozzle, strip velocity V, etc.
As a method of coating weight adjustment to be made in the course of molten metal plating, methods disclosed for example in Japanese Patent Laid-Open No. Sho 54-149331, Japanese Patent Publication No. Sho 56-12316 and Japanese Patent Laid-Open No. Hei 1-92324 have been known.
In Japanese Patent Laid-Open No. Sho 54-149331, a gas pressure on the surface of a strip is considered to be a function of a distance from a plating bath surface, and nozzle height, nozzle-strip distance, and gas injection pressure are adjusted to satisfy the function; in Japanese Patent Publication No. Sho 56-12316, the coating weight of plating is adjusted by utilizing the gas injection pressure expressed as the function of the distance, height and angle of nozzle, line speed, and coating weight of plating; and in Japanese Patent Laid-Open No. Hei 1-92324, the coating weight of plating is adjusted by using a relational formula of a wiping pressure and coating weight and a relational formula of a nozzle distance and coating weight, at the time of feedback control of coating weight of plating immediately above the nozzle.
There have been generally practiced such a method of feedback control of the coating weight of plating (deposit of coating material) based on wiping nozzle injection pressure P, nozzle-strip distance D, strip velocity V, and molten metal coating weight W, and a method of feed-forward control by using a relational formula of the molten metal coating weight W and operation factors.
Generally, when the feed-forward control or feedback control is carried out, the wiping nozzle injection pressure P and the nozzle-strip distance D are determined on the basis of the relational formula of the coating weight W of the coating material and operation factors in accordance with a target coating weight for the purpose of adjusting the coating weight of the coating material such as the molten metal, paint, etc. to be applied to a continuously moving strip. Therefore, in order to accurately adjust the coating weight of the coating material, it becomes important that the above-described relational formula to be used for control can properly and accurately express the wiping phenomenon of molten metal or paint throughout the range of operation.